I have had a few of these and have had mixed results.  I've done a lot of reading on them and know many people who run them, some commercially.  They are used in a wide variety of commercial equipment aside from the Simplicity's.  Overall, I think all you will read online is:

A:  Have one and love the power

B:  Overheating, blown engine complaints

Generally, most folks only take the time to write feedback online when something goes wrong, especially in the forums.  I picked up a legacy with thousands of hours on it, and the diesel ran great.  Certainly seemed like a motor to outlast the tractor.  I also had one with 500hours on it that ate the lower end bearings and crank.  I got an outside opinion on it, and they believe it was neglected and starved for oil at some point.  No indication of overheating, no plugged galleries, good oil pump, etc.  

I doubled down and bought another one with the diesel, and despite my misfortune on the most recent one before it, expect a very long life.  They sip fuel, are easy to maintain and the power is way more than what the tractor needs.  I run a flail mower, mid mount mower, tiller, conical log splitter, loader, 4' blower, 5' broom, etc and it's got gobs of power.  The liquid cooled gas motors are quieter and smoother, but the diesel really should outlive them.